Description of problem:
dnf-plugin-spacewalk launches even if client is not registered into Spacewalk which leads to error in DNF which ends abnormally.

Version-Release number of selected component (if applicable):
dnf-1.0.0-1.fc22.noarch
dnf-plugin-spacewalk-2.4.11-1.fc22.noarch

How reproducible:
100%, tested on 1 system

Steps to Reproduce:
1. install dnf-plugin-spacewalk
2. dnf install $PACKAGE

Actual results:
Traceback (most recent call last):
  File "/usr/bin/dnf", line 36, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 185, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 84, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 115, in _main
    cli.configure(map(ucd, args))
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1016, in configure
    self.base.plugins.run_config()
  File "/usr/lib/python2.7/site-packages/dnf/plugin.py", line 82, in fn
    dnf.util.mapall(operator.methodcaller(method), self.plugins)
  File "/usr/lib/python2.7/site-packages/dnf/util.py", line 157, in mapall
    return list(map(fn, *seq))
  File "/usr/lib/python2.7/site-packages/dnf-plugins/spacewalk.py", line 75, in config
    self.activate_channels(self.cli.demands.sack_activation)
  File "/usr/lib/python2.7/site-packages/dnf-plugins/spacewalk.py", line 94, in activate_channels
    login_info = up2date_client.up2dateAuth.getLoginInfo(timeout=self.timeout)
  File "/usr/share/rhn/up2date_client/up2dateAuth.py", line 227, in getLoginInfo
    login(timeout=timeout)
  File "/usr/share/rhn/up2date_client/up2dateAuth.py", line 178, in login
    server = rhnserver.RhnServer(timeout=timeout)
  File "/usr/share/rhn/up2date_client/rhnserver.py", line 171, in __init__
    timeout=timeout)
  File "/usr/share/rhn/up2date_client/rpcServer.py", line 171, in getServer
    raise up2dateErrors.SSLCertificateFileNotFound(msg)
up2date_client.up2dateErrors.SSLCertificateFileNotFound: rhn-plugin: ERROR: can not find RHNS CA file: /usr/share/rhn/RHN-ORG-TRUSTED-SSL-CERT

as system is not registered into Spacewalk there's not RHNS CA file which leads to previous error

Expected results:
dnf should complete action normally with dnf-plugin-spacewalk disabled